The invention relates to a method of drape drawing, by means of a drawing process, a shadow mask sheet for a color display tube consisting of a nickel-iron alloy.
A colour display tube usually comprises an envelope having a glass display window which has a display screen with phosphor regions luminescing in the colors red, green and blue. A shadow mask having a large number of apertures is mounted in the tube a short distance from the front of the display screen. During operation of the tube three electron beams are generated in the tube by an electron gun system, pass through the apertures in the shadow mask, and impinge onto the said phosphor regions. The mutual positions of the apertures and the phosphor regions are such that, upon writing the picture, each of the electron beams always impinges on phosphor regions of one color. However, a considerable proportion of the electrons impinge on the shadow mask, the kinetic energy of these electrons being converted into thermal energy so that the temperature of the shadow mask rises. The thermal expansion of the shadow mask associated with this rise in temperature may result in a local or complete bulge of the shadow mask as a result of which the mutual positions of the apertures in the shadow mask and the phosphor regions associated with these apertures are interfered with. This results in color defects in the displayed picture, which become more serious as the shadow mask is less convex. This flatter shape and resulting problem is found more and more in the present generation of color display tubes having flatter display windows.
It is known to mitigate such problems caused by thermal effects by manufacturing the shadow mask from a material having a low coefficient of thermal expansion. An example of such a material is an alloy of substantially iron and nickel in which the nickel content is approximately 36% by weight. The high tensile strength and hence difficult machinability of these alloys have hampered their use as shadow mask materials. A difficult machinability of the material generally leads to a rapid detrition of the drawing tools with which the shadow mask sheet is drape drawn. Moreover, the reproducibility of the drawing process decreases as a result of detrition of the drawing tools. A rapid detrition hence requires an intensive control and frequent maintenance of the drawing tool.
This detrition problem is the more prominent when, during the drawing process, a shadow mask sheet is clamped in a slipping manner over at least a part of its circumference. Since the shadow mask sheet is subjected to the drawing process after a pattern of apertures has already been provided therein, the tensile strength of the sheets will generally be different in mutually perpendicular directions. In order to prevent the shadow mask from being drawn to pieces during the drawing process in the direction of the smallest tensile strength, it is clamped in a slightly slipping manner in the direction of the smallest tensile strength. The frictional forces occurring during the resulting slipping movement should be reproducible in value so as to obtain a reproducible drawing process. As a result of detrition which is promoted by large frictional forces, the frictional forces no longer occur in a reproducible manner; and as a result the reproducibility of the drawing process also decreases.